There are known dampers that are interposed on a torque transmission path that transmits torque of an engine to reduce torsional vibration that is contained in torque transmitted. Dampers of this type include not only a clutch damper that functions as not only a damper but also a clutch (refer to JP-A-2009-228736, for example) but also a dual mass flywheel that includes a damper and two flywheels (refer to International Publication No. 2010/010896, for example), in addition to a single function damper that functions only as a damper.
FIGS. 8 to 11 show a conventional dual mass flywheel 1P, and this dual mass flywheel 1P includes a first flywheel 2, a second flywheel 3, coils springs 4 and spring seats 5.
The first flywheel 2 is a rotary member that is fixed to a crankshaft (not shown) of an engine via bolts B and includes a flywheel portion 21, a seal plate 22, a hub 23, a bearing support plate 24 and a ring gear 25.
The flywheel portion 21 has a disc-shaped inner circumferential wheel portion 21a, two first protuberant portions 21b that are formed along a circumferential direction on an outer circumferential side of the inner circumferential wheel portion 21a, two first non-protuberant portions 21c that are formed on the outer circumferential side of the inner circumferential wheel portion 21a so as to be positioned between the two first protuberant portions 21b in the circumferential direction, and a cylindrical portion 21d that is provided so as to extend in an axial direction from an outer circumferential edge portion of the first protuberant portions 21b and the first non-protuberant portions 21c. The first protuberant portions 21b protrude further outwards than the inner circumferential wheel portion 21a and the first non-protuberant portions 21c, and accommodation spaces S for the coil springs 4 and the spring seats 5 are defined inside the first protuberant portions 21b. 
The seal plate 22 has an inner circumferential plate portion 22a, two second protuberant portions 22b that are formed along a circumferential direction on an outer circumferential side of the inner circumferential plate portion 22a, two second non-protuberant portions 22c that are formed on an outer circumferential side of the inner circumferential plate portion 22a so as to be positioned between the two second protuberant portions 22b in the circumferential direction, and a fixed portion 22d that is fixed to a distal end of the cylindrical portion 21d on an outer circumferential side of the second protuberant portions 22b and the second non-protuberant portions 22c. The second protuberant portions 22b protrude further outwards than the inner circumferential plate portion 22a and the second non-protuberant portions 22c, and the accommodation spaces S are defined inside the second protuberant portions 22b. 
The hub 23 is a cylindrical member that is fixed to a central portion of the flywheel 21 and is fixed to the crankshaft together with the flywheel portion 21 with the bolts B. In addition, a bearing 27 is fitted on an outer circumferential portion of the hub 23, and the second flywheel 3 is supported rotatably on the first flywheel 2 via the bearing 27.
The bearing support plate 24 is a member that restricts an axial movement of the bearing 27 and is fixed to the crankshaft together with the flywheel portion 21 and the hub 23 with the bolts B. The ring gear 25 is a ring-shaped gear member that is fixed to an outer circumferential portion of the cylindrical portion 21d. When the engine is started, the ring gear 25 meshes with an output gear (not shown) of a starter motor to transmit engine starting torque to the crankshaft via the first flywheel 2.
The two accommodation spaces S are defined in the circumferential direction in the interior of the first flywheel 2. Each accommodation space S is a space extending along the circumferential direction which is surrounded by the first protuberant portion 21b, the second protuberant portion 22b and the cylindrical portion 21d, and both circumferential end positions thereof are defined by the first non-protuberant portion 21c and the second non-protuberant portion 22c. A first abutment portion 26, which is formed by a step portion that connects the first protuberant portion 21b and the first non-protuberant portion 21c together and a step portion that connects the second protuberant portion 22b and the second non-protuberant portion 22c together, is provided at each end of each accommodation space S.
The second flywheel 3 is a rotary member that is fixed to a transmission (not shown) and has a driven plate 31 that is disposed in the interior of the first flywheel 2 and a flywheel portion 32 that is disposed outside the first flywheel 2. The driven plate 31 and the flywheel portion 32 are connected together with a plurality of rivets 33 and supports rotatably the first flywheel 2 via the bearing 27.
The driven plate 31 has a disc portion 31a and two extended portions 31b that are extended radially outwards from an outer circumferential portion of the disc portion 31a. The disc portion 31a is disposed further radially inwards than the accommodation spaces S in the interior of the first flywheel 2, and the two extended portions 31b are extended from an outer circumferential portion of the disc portion 31a towards the interiors of the accommodation spaces S. The two extended portions 31b are formed so that their positions are offset 180° in the circumferential direction, and a second abutment portion 36 is formed at both circumferential end portions of each extended portion 31b which extends radially and axially and which is at right angles to the circumferential direction.
The coil springs 4 are arranged with a posture in which they follow the circumferential direction in the accommodation spaces S to transmit torque between the first flywheel 2 and the second flywheel 3. The spring seats 5 are disposed between the coil springs 4 and the first abutment portions 26 or the second abutment portions 36 on both sides of a rotational direction of the first flywheel 2 and slide on an inner circumferential surface 21e of the cylindrical portion 21d as the first flywheel 2 and the second flywheel 3 rotate relatively.
As shown in FIG. 11, in the dual mass flywheel 1P in an initial state, the spring seats 5 on both the sides of the rotational direction of the first flywheel 2 are in elastic abutment with the first abutment portions 26 and the second abutment portions 36. As shown in FIG. 12, when the vehicle is decelerated, although the first flywheel 2 rotates in a counterclockwise direction (refer to an arrow in the figure), since the rotation speed of the second flywheel 3 that is connected to the transmission is relatively faster than the rotation speed of the first flywheel 2 that is connected to the engine, the second abutment portion 36 of the second flywheel 3 that is positioned on a counter-rotating direction side relative to the coil spring 4 (a right side in the case of the coil spring 4 that is positioned at an upper part of the figure) presses the first abutment portion 26 of the first flywheel 2 that is positioned on a rotating direction side relative to the coil spring 4 (a left side in the case of the coil spring 4 that is positioned at the upper part of the figure) via the spring seats 5 and the coil springs 4, whereby torque is transmitted from the second flywheel 3 to the first flywheel 2. In FIGS. 12, 13 and other figures (FIGS. 2 to 5), relative positions of the first flywheel 2 and the second flywheel 3 are shown. When torque is so transmitted, the coil springs 4 are expanded or contracted in response to fluctuation in torque transmitted, whereby torsional vibration contained in the torque transmitted is reduced.
In this type of dual mass flywheel 1P, however, there may be occurring a phenomenon in which the spring seats 5 are caused to stick to the inner circumferential surface 21e of the first flywheel 2 by centrifugal force. When the vehicle is accelerated from the state shown in FIG. 12 in which the vehicle is decelerated, the first flywheel 2 is caused to rotate relatively faster than the second flywheel 3. Consequently, the first abutment portion 26 of the first flywheel 2 that is positioned on the counter-rotating direction side relative to the coil spring 4 (the right side in the case of the coil spring 4 that is positioned on the upper part in the figure) passes the second abutment portion 36 of the second flywheel 3, and the first abutment portion 26 of the first flywheel 2 starts to press the second abutment portion 36 of the second flywheel 3 that is positioned on the rotating direction side (the left side in the case of the coil spring 4 that is positioned at the upper part of the figure) via the spring seats 5 and the coil springs 4.
When the vehicle's state is shifted from the decelerated state to the accelerated state, as shown in FIG. 13, in case the spring seat 5 sticks to the inner circumferential surface 21e of the first flywheel 2, a clearance D is generated between the spring seat 5 that is positioned at the counter-rotating direction side end portion and the second abutment portion 36 of the second flywheel 3 that is positioned on the counter-rotating direction side, and there are fears that an impact generated in eliminating the clearance D represents a cause for generation of abnormal noise and vibration. Namely, when the vehicle's state is shifted from the decelerated state to the accelerated state, it should be natural that the spring seat 5 on the counter-rotating direction side slides over the inner circumferential surface 21e of the first flywheel 2 to be brought into abutment with the second abutment portion 36 of the second flywheel 3 and that with the spring seat 5 kept in abutment with the second abutment portion 36 of the second flywheel 3, the first abutment portion 26 of the first flywheel 2 passes the second abutment portion 36 of the second flywheel 3 and is then brought into abutment with the second abutment portion 36 of the second flywheel 3 to thereby be pressed. However, due to sticking to the inner circumferential surface 21e of the first flywheel 2, the first abutment portion 26 of the first flywheel 2 does not follow the second abutment portion 36 of the second flywheel 3 (a distance L in FIG. 12 is maintained), and the clearance D is generated between the first abutment portion 26 and the second abutment portion 36 when the first abutment portion 26 of the first flywheel 2 passes the second abutment portion 36 of the second flywheel 3.
In particular, in the dual mass flywheel 1P that can ensure a good vibration reduction performance over a wide torque fluctuation range from low torque to high torque, three or more coil springs 4 are disposed in series in each accommodation space S, and the spring constant of the coil springs 4 that are positioned at the circumferential ends is set smaller than the spring constant of the remaining coil spring 4, this facilitating the generation of a sticking phenomenon of the spring seats 5.